Remote indicating compass



March 2, 1948. A. T. slNKs 2,437,132

REMOTE INDICATING COMPASS Filed Nov. 1'7, 1943 3 Sheets-Sheet l Tnx/enter: AHen T Sfks,

*by His Attorney.

TRANsMxTTER March 2, 194s. A. T, SINK'S 2,437,132

REMOTE INDICATING COMPASS Filed NOV. 17, 1943 3 Sheets-Sheet 2 Inventor: AHeYw T Sinks,

by v

His Attorney.

March 2, 1948. A. T. slNKs 2,437,132

REMOTE INDICATING COMPASS I Fil'ed Nov. 17, 1943 I5 Sheets-Sheet 5 If Mu Fia.

T|ME Mu Fig. 9. Iz

Ez` V Mu' M u F1210. f2

Mu Figli.

by 7?/ JM/MM His Attofney.

lPatented Mar. 2, 1948 2,437,132 REMOTE INDIcArmG COMPASS Allen T. Sinks, Beach Bluff, Mass., assigner to General Electric Company, a corporation of New York Application November 17, 1943, Serial No. 510,614

4 Claims. 1

My invention relates to a remote indicating compass, and its object is to provide such apparatus suitable for use on airplanes and wherein errors which would otherwise exist in the transmitter are eliminated.

The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of my invention, reference is made in the following description to the accompanying drawings in which Fig. l shows a side view partially in section of a compass transmitter embodying my invention; Fig. 2 shows the electrical connections of transmitter and receiver; Figs. 3, 4 and 6 represent undesirable arrangements of compass transmitter parts for the purpose of explaining errors compensated for when the same parts are rearranged according to Figs. 1, 2 and 5; Fig. 7 represents a plan view of compass magnet and transmitter core; Fig. 8 shows curves of the fundamental excitation voltage, excitation current, and excitation iiux of the transmitter; Fig. 9 shows curves of the average permeability variation in the transmitter core, together with resulting second harmonic generated volts and current curves in the transmitter, characteristic of an undesirable, low power factor, second harmonic circuit condition; Fig. 10 shows curves corresponding to those of Fig. 9, but for a power factor condition modiied in accordance with one aspect of the present invention and with the permeability curve of the receiver added; and Fig. 11 shows the permeability curve of the transmitter and the second harmonic voltage and current curve of the receiver for such modiiled power factor, second harmonic circuit condition.

Referring to the drawings, and particularly to Fig. 1, I have shown a suitable casing I of nonmagnetic material having a liquid tight inner chamber I I. Such chamber is adapted to contain a suitable liquid for largely supporting a compass card structure consisting of a iioat I2, damping wings I3, and a pair of permanent magnets I4. A plan view of the magnets is shown in Fig. 7. The compass card structure includes a pivot I5 resting on a jewel bearing I6 so as to permit rotation of the compass card structure as well as tipping movement relative to the jewel in order that when the airplane or other structure on which the compass is mounted departs from a level position, the compass card which is designed to be pendulous will remain with the axis of its magnets I4 substantially level. The jewel bearing is supported on a Arod Il slidably mounted in a cylinder I8 and urged upward by a spring I9. Sliding movement of the rod is permitted between the limits determined by collars 20 on the rod and a stop screw 2l. This provides a cushion support for that part of the weight of the compass card structure not supported by the liquid, and prevents injury to the pivot bearing in case of shock and vibration. A collar 29 prevents the compass card structure from becoming unseated by reason of inversion of the apparatus. A resilient bellows closes the top of the inner chamber to permit expansion and contraction oi the liquid. The inner chamber is lled with kerosene oil, for example, and supports the greater part of the weight of the compass card structure. The baiiles I3 serve as damping means. The top of the casing may be protected by a cap |22.

In the operation of this device the magnets I4 align themselves with the earths magnetic iield, but they are made appreciably stronger than is necessary for ordinary compass operation in order that the compass card structure may serve as the rotor of an electromagnetic telemetering transmitter for transmitting the compass reading to one or more remote points. Thus the magnets I4 simultaneously serve a double function. The stator of such transmitter consists of an annular core 22 of high permeability but saturable magnetic material, such as Mumetal, on which are wound an exciting Winding 23 and a coupling winding having two groups of coils 24 and 25 (see Fig. 2). It is seen that the stator is mounted well belowthe position of compass magnets I4 and vis supported in the base of the casing concentric with .thewaxis of pivot rod Il and parallel to 'thelmaghets I4 when the compass card and casingg'a're in alevel position represented in Fig. 1. The. stator is firmly supported in place at three or more points by suitable supporting structure, such as is represented at 25. The bottom of the casing may be closed by a protective closure 2l, and the leads for the windings may be brought out to plug connections, one of which is indicated at 28.

The telemetering system employed utilizes the second harmonic principle of application Serial No. 461,710, led October l2, 1942, to Hendrik D. Middel, Position indicator, assigned to the same assignee as the present invention and which case has become abandoned. Both the transmitter represented at the left in Fig. 2 and the receiver represented at the right in Fig. 2 have primary exciting and polyphase secondary coupling windings wound on a stationary ring core. The pri mary exciting winding of thereceiver is indicated at 3D, and, like the primary exciting winding of the transmitter, is uniformly distributed over the complete ring core represented at 3l.

These windings are energized in parallel from av constant frequency source of alternating current 33 and produce alternating current fluxes first in one direction and then in the other about their ring cores. These windings are wound and connected to produce iluxes in the same direction simultaneously in the two devices. Except for the overlapping relation of the secondary coupling coils 24 and 25 on thetransmitter, said relation not always being essential on the receiver, the coupling coils on the receiver may be` considered to be similarv and comprise the pair 34 corresponding to coils 24 of the transmitter and the pair 35 corresponding to coils 25 of the transmitter. The coupling coils of each vpair are wound on diametrically opposite sides of their cores and are connected in series opposition. Thus coils 34 of the receiver are on opposite sides of the core, are connected in series, and, if energized, would produce bucking fluxes about core 3l. The two pairs of coupling coils on transmitter and receiver are displaced in quadrature relation. Thus coils 34 are in quadrature displaced relation with respect to coils 35 on corev 3i. The similarly placed pairs of coupling coils on transmitter and receiver are connected in series opposition. Thus coils 2li on the transmitter are connected in series opposition to coils 34 on the receiver. Coils 25 on the transmitter are connected in series opposition with coils 35 on the receiver. In interconnecting the transmitter and receiver, I may make use of the alternating current supply lines 35 and 3l which run between them. The interconnection between coils 25 and 35 may be traced as follows: From the receiver upper coil 35 through wire 35 to the transmitter upper coil 25 through lower coil 25, and through wire 38 back to the lower coil 35 of the receiver. Similarly the coils 24 and 34 make use of the other supply wire 3l and the wire 39 for their interconnection. It is possible to thus utilize wires 35 and 3l for the double purpose, since there is no interconnection between these wires through the two groups of coupling coils and their connections forming the polyphase interconnecting system between transmitter and receiver.

The rotating element of the transmitter is the previously mentioned compass card structure of Fig. l, the compass magnets of which are represented at i4 in Fig. 2, with supporting structure which may diagrammatically represent the oat supporting structure i2 of Fig. l. The particular shape of the magnets i4 as shown is not essential. The rotating element of the receiver comprises a pair of parallel permanent magnets 40 suitably supported for rotation concentrically within the receiver stator element by a member- 4|. The stator and rotor parts of the receiverare structurally considerably smaller than those of the transmitter. The receiver is designed with a view of being mounted at -any angle on a crowded instrument panel, while the 'compass transmitter may be mounted in an airplane wing or otherwise where there is ample room and a minimum of disturbing inuence. The magnets of the compass card align themselves with the'earths eld, as indicated in Fig. 2, and also producea unidirectional iiux in the core of the transmitter, as indicated in dotted lines in Fig. 2. This flux combines with the alternating ux in the core produced by the exciting winding 23 to produce a saturated condition of the core on one side which shifts to the opposite side with reversal of the alternating current iiux. Thus in Fig. 2 when the alternating current iiux is in the direction represented by arrows- 42 and 43, that side adjacent arrow 43 is saturated land when the alternating current flux reverses, the side adjacent arrow 42 becomes saturated.

Thevoltages induced in the reversely connected coupling coils of a pair in the transmitter are therefore no longer equal and opposite but uning double frequency-voltage is induced lx1-the' coupling coils. Corresponding currents fiow in the coupling coils of the receiver and react with the excitation ux of the core thereof to produce a directional effect in flux with which the rotor magnets of the receiver align themselves to reproduce the indication of the transmitter compass card. Relative rotation between stator and rotor` of thev transmitter is reproduced in the receiver. The present invention is not concerned with this principle -of telemetering but rather with features for vthe elimination of certain errors.

One source of error is the deviation of the transmitterA magnet i4 from a normal N and S axis due to very slight but unavoidable dissy'nmetry in the shape and position of the ring core 22. The heid created by the magnet in the vicmity of the ring core 22 is so much greater than the earths eld acting on the magnet that a very small unbalance of the former will cause considerable deviation of the magnet from the normal position it assumes in the earths field. This error, if present, is compensated for by the use of a small elongated strip of magnetic material 44, Fig. 1, positioned within the ring core 22 and adjustable relative to the axis thereof so that its longitudinal axis may be rotated in any desired direction. The magnitude and direction of the' magnetic unbalance of the ring core determines the size, position, and orientation of strip fili.

Another source of error of the same character arises from a tilting of the ring core due to tilting of the-plane on which itis mounted. While it is theoretically possible to mount the ring core and compass magnet on gimbals and pivots so that both will remain horizontal, it is much more practicable to allow the ring core 22 to tilt with the plane, or other structure on which it is mounted. I have found that if the magnets i6 are relatively far away from the ring core 22, the eect of tilting of the ring is very slight up to tilts of the order of 10 degrees. However. a large spacing results in low output from the transmitter as well as a large case to contain the two elements on account of the spacing between them. I have found that the magnets i4 can be mounted, relatively nearer the ring core 22 as represented in Fig. l without producing much error due to tilt, if a certain relation between the position of the pivot i5 above or below the magnet i4 and the position of the magnet above the ring 22 is maintained. For instance, for magnet and ring of the relative dimensions indicated in Figs. 3, 4, and 5, using a ring core 22 of 3% inches outside diameter and magnets 2%.; inches long x .265 inch in diameter spaced 0.8 inch apart, it was found that the error due to tilt was reduced to a very small degree when the center of the magnets was 1% inches above the center of the ring and the pivot 1A inch below the center of the magnet. and a similar condition was obtained when the'v center of the magnets was 1% inches above the center of the ring and the pivot point l/4 inch above the center of the magnets. The latter arrangement is represented in Fig. 5. Other relations of ring diameter, length of magnet, spacing of magnet and ring and point of pivot of magnet would be satisfactory but due to the complications involved in calculating such relations, they should be checked by actual trial before adoption. The reason for the tilt error and for its elimination in the example given can be explained in connection with Figs. 3 and 4, 5 and v'7. Figs. 3 and 4 being arrangements subject to an intolerable tilt error and 'included for explanatory purposes only. lin Fig. 3 it is evident that, with the magnets I4 mounted in the plane of the ring magnetic core 22, `if the ring is tilted, the magnet will tend to tilt with it as indicated. The magnets will also tend to swing into position 90 degrees in a horizontal plane from the position shown as the reluctance between the magnets and tipped ring is lowest in that position. It is therefore seen that when the axis about which the ring is tilted is 45 degrees to the north and south line, there will be a large deviation of the magnet from its correct position. On the other hand, if the magnet is mounted above the ring and pivoted at the center of the ring in such a manner that the center of the ring remains iixed relative to the center of the magnet and the ring is tilted as indicated in Fig. 4, the air gap between the magnet and the ring onthe left will be considerably less Ithan that on the right and the magnet will be tipped to the left as represented making the 'disparity of the gaps still greater, so that the tipping effect of the magnetic forces will be further magnified, the final angle of tilt oi the magnets being established when the pendulous torque equals the magnetic torque. Due to the nonlinear relation of the reluctance between the magnet poles and the ring relative to the .gap spacing, the total reluctance of the magnetic` circuit of the magnet for a given average of the two gaps is a maximum when the gaps are equal. Therefore, this total reluctance will be a minimum for a given tilt angle oi 'the ring when the angular position or the magnet in a horizontal plane is as indicated in Fig. 'l where x-x represents the ring tilt axis, so that again when the axis about which the ring is tilted is 45 degrees to north and south, there will be a large deviation of the magnet from its correct heading due to its tendency to assume a position perpendicular to the axis about which the ring is tilted. This tendency is opposite to V 'that described in connection with Fig. 3. Therefore, it would be expected that some pivot position intermediate between that of Fig. 3 and that of Fig. would eliminate the error. That such a surmise is sound is borne out by experiment as well as by the following analysis. In Fig. 5 the ring in eiiect rotates about the pivot point of the magnet as a center when it is tilted (tilted position or ring 22, Fig. 5), and if this pivot point is raised to the proper position, then when the ring tilts to the same angle as previously considered, the center of the ring is in eect displaced to the left. This has two eiects. First, the gap on the left gl is increased and that on the right gr is decreased as compared to Fig. 4. Furthermore, the component of force downward due to the magnetic pull on the left is a smaller percentage of the totaldue to the change of angle, whereas that on the right becomes slightly larger. The net result, with the proper position of thepivot I5, is to keep the downward components oi? force on the two ends of the magnet practically equal. Therefore the magnets remain essentially level and the difference between the gaps is still further reduced below that obtained with the ar- 2. TA=4N2K sine Vr sine V0 rangement of Fig. 4. The net result is that the total reluctance of the magnet circuit of the permanent magnets in the tilted position of the ring is essentially the same when the ring is not tilted, and also essentially the same as it is when the ring is tilted and the magnet is rotated through a horizontal angle of degrees from the position represented in Fig. 5. Any tendency, therefore, of the magnet to assume any particular position relative to the axis of the tilt of the ring is reduced to a minimum, and as a matter of practice, to an almost negligible quantity. By such arrangement tilting of the ring does not cause a compass error in the position of the magnet.

There are other minor sources o error associated with the tilt of the ring and the vertical component of the earths iield; one which may be called a Ileld distortion error and the other a dip error. These errors can be made to substantially cancel each other by a proper choice of the pivot position, one such position being that represented in Fig. l, corresponding closely to the arrangement of Fig. 5 in these respects.

If the transmitter has its coupling coils spaced apart as :represented for the coupling coils 34 and 35 of the receiver, or if the ends of such coils merely abut each other, it is found that the iiux of the rotor magnets i4 produces in conjunction with the current in the coupling coils a pronounced locking tendency when the rotor is in such position that its magnetic axis bisects the dividing line between the coupling coils. "ihus if the coils 24 and 25 of the transmitter did not overlap but were separated like coils 34 and 35 of the receiver, the transmitter would have unstable zero points in its torque curve for the rotor position shown in Fig. 2 and for rotor positions 90, ist), and 27u degrees from the position shown. lt would have stable or locking points in its torque curve in positions 45 and 135 degrees in either direction from the position shown. Such locking torque may be reduced to an undetectable minimum by having the coils 24 and 25 of the transmitter span an -arcof degrees and to overlap uniformly as represented. The reason for such locking torque and for its elimination by such-overlapping can be explained by mathematical formula as follows:

it it is assumed that the magnet is symmetrical, then the ux from the magnet entering the magnetic ring, when not saturated, can be represented loy the following formula:

Let B=ux per unit length of arc which enters the ring at some location on a radius B, Fig. 6, making an angle :c with the axis of the magnet,

1. Then B=B1 cos .'c-i-Ba cos 3:12-1-B5 cos 5m Where B1, B3, etc. are coeflicients of the harmonies in a particular Fourier series.

If it is further assumed that the average torque-producing current in coils A, Fig. 6, is proportional to the net ilux linkages in these coils produced by the ux B, then the torque exerted on the magnet by the A coils turns out tobe:

BT? sine Ur sine U0 Val U-l Where V and U are odd integers. This means that the expression in each bracket is the sum of a series in which V takes on all the values of the coeiiicients of a: in Equation 1; in other words, 1, 3, 5, 7, etc., and similarly for U, except that V and U vary independently. Therefore. the expression in the-lrst set of brackets of Equation 2 can be written:

B, sine :t sine sine 3x sine 30+ gaine 5x59 etc.

and, similarly, for the second set or brackets. The only difference between the expressions in the first and second brackets of Formula 2 is that one has the parameter V2 in the denominator, and in the other the corresponding parameter U occurs to the first power. In Formula 2, r=1/2 the angle covered by one coll, Fig. 6.

N is the number of turns.

K is a constant of proportionality whose value de- I pends on the impedance of the coil circuit, the radius of the ring, and the units employed.

V and U are coeiicients which may have any of the values of the coelcients of m in Formula 1.

Bv or BU=B1, B3, or Bri-in Formula 1 when V or U== 1, 3, 5, respectively.

The value of torque produced by the D coils, Fig. 6, can be obtained immediately from Formula 2 by substituting 7ll (Ha) When this is done and the two torques are added together, the resulting torque To considering all harmonics up to and including the seventh is given by the following:

3. Toa-410K (12113 sino r sine 3f 5541+513) sino 46 quence in the ilux distribution are the third and the ilfth, and even the nfth is relatively slight. Therefore, the only terms of much consequence in the expression for To, Formula 3. are the rst two, of which the first is by far the greater. One of the coeicients appearing in the ilrst term is the sine of 3r. Since r is half the -angular span covered by the coil, i! the coil covers 120 degrees of a circle, r is and thesine of 31- is zero. Moreover, the sine of r is .865 as is likewise the sine of 5r. Therefore, by making the coil angle 120 degrees, the first termin therighthand side of Equation 3, by far the largest, is entirely eliminated; and the second term which is small anyway is somewhat reduced. While this theory is somewhat involved, it may be stated that in actual practice the large locking l torque occasioned by a coupling coil arrangement, such as is illustrated in Fig. 6, in the transmitter was reduced to an undetectable quantity by making the coupling coil span cover 120 degrees, as illustrated in Fig. 2. This feature is generally 'applicable to second harmonic type of transmitters and is not dependent upon the 8A nature ci the indication to be transmitted or the particular means for producing the unidirectional ilux ield of the transmitter rotor.

Another kind of error in the transmitter may be introduced by the receiver because if the transmitter and receiver excitation circuits are made alike with respect to power factor angle. any relative angular displacement between the transmitter magnets and the receiver magnets will cause torques of approximately equal magnitudes to be produced in each device. Consequently, any torque necessary to overcome friction or inertia in the receiver requires an equal and opposite torque of the transmitter, which is thereby deected from its true heading by an amount depending on the ratio between this feedback torque and the stiness oi coupling between the transmitter magnets and the earths magnetic ileld. Hence, for a given friction torque in the receiver, for instance, there would be two errors: (l) the angular displacement between transmitter and receiver necessary to cause the necessary coupling torque between the two devices, and (2) the angular displacement of the transmitter magnets fromtheir correct heading caused by the torque acting on the transmitter. Moreover, the direction of these two errors is such that they would be additive. The signiiicance oi this additional error becomes still more important when more receivers than one are operated simultaneously from one transmitter, which is possible with the-arrangement herein described. In this case not only is the deflection of the transmitter magnets from the true heading increased due to larger load, but also any sticking or mechanical interference in one receiver moving system will affect the other receivers seriously on account of the verror of heading oi' the transmitter element occasioned by the faulty indicator. This effect is particularly important on military aircraft wherey one of the receivers may be damaged in combat.

By properly adjusting the impedance phase angle of the coupling winding system, Fig. 2 previously described, and also adjusting the phase angle oi' the exciting currents in either or both the transmitter and receiver cores, I have found that the torque the receiver produces on the transmitter can be reduced to substantially zero without appreciably reducing the torque which the transmitter produces on the receiver, and in fact, the torque of the receiver can actually be increased. The following analysis explains how the proper torque relations can be obtained.

While it is possible to make a system of the type described here having locking torques, it is one of the objects of this invention to eliminate such locking torques as have been described previously herein by the overlapping of the coupling coils of the transmitter, and therefore, it will be assumed in the following analysis that locking torques do not exist. It will further be assumed that each instrument by itself, when its coupling coils are loaded with-fixed equal impedances, or even short circuited, will have no locking torques. It will also be assumed that when the transmitter and receiver are operating together, the second harmonic circuit impedances are suillciently constant so that the total second harmonic current can be regarded as the time-vector sum of where Ecl is the second harmonic voltage gen' in sa Z Z `have the same phase, and their relative magnitudes depend on the angular position of the transmitter magnets, and conversely, these two currents establish an alternating current eld in the region of the magnets whose direction depends on their relative value. Moreover, it can 4be shown that if there is no locking,

and

where I2 is the maximum valuel of ne ne Z Z and is the angular heading of the magnets or" the transmitter, with respect to the axis of the ne Z component. Therefore, the direction of set up by the two components oi current will be the same as the heading of the transu niitter magnet, and the magnitude or the field will be proportional to the square root of the sum of the squares of these currents, or a constant Kia, where K is the proportionality constant. in a similar manner the same currents produce a neld of greater or less magnitude, depending on the design, but of exactly the saine heading in the receiver. Therefore, the currents can be considered as a combined single current le, whose average value with respect to time is constant but which iiows in two coils, one in the transmitter and one in the indicator, whose headings or axes are the same as the heading o1 the magnets producing the current, in this case the transmitter magnets. Consequently, this current l2 never produces any torque on the transmitter because the eld corresponding to the current is in line with the transmitter magnets, whereas torque is produced on the receiver magnets as soon as their heading departs from that of the transmitter magnets.

In a similar manner a set of currents I2' will be generated by the receiver which will produce no torque on the receiver but will produce torque in the transmitter when the headings of the magnets of the two instruments differ.

The torques thus far considered are torques alternating with respect to time and would have an average value of zero, if the permeabilities of the magnetic circuits were constant with respect to time. However, a study of Fig. 8 shows how is equal to I2 sine 6, and EZ@ is equal to I2 cos 6 and the ield and ` saturation of the cores results in the production of a net average torque. Curve I2, Fig. 9, shows the combined second harmonic current generated by the transmitter and the voltage E2 that causes Iz to now. This voltage is produced by the change in the amount of the permanent magnet ilux linking the coupling coils, said change in flux being caused by the change in permea- ,fbility mu, of the core, resulting from the fundamental excitation curve If, Fig. 8. Since E2 is proportional to the rate of change of :flux En is degrees on the second harmonic base, or 45 degrees on the fundamental base behind the phase of permeability curve mu. For the sake ci completeness, the fundamental excitation frequency curve Vf and the fundamental excitation flux curve 6x are included in Fig. 8. Figs. 8 and 9 could 'be made as one figure but the curves are separated, as shown, for the sake of clarity, but the two sets of curves are plotted to the same time coordinates. The curve mu is the same in Figs. 8 to 11. Fig. 9 is illustrative of a low power factor second harmonic circuit, the curve I2 being almost 90 degrees in time phase behind the voltage producing it, and in general represents what could be expected under ordinary conditions with no power factor adjustments of the circuits. f, now, the power factor of the receiver excitation circuit is made identical with that oi' the transmitter, the phase o the penne ability variation and the phase o the second nar= monic voltage and current resulting lfrom it will loe the same as in Fig. 9. Hence, insofar as time phase (not direction in space) is concerned, E2 could also be considered the current produced by the receiver, and mu the permeability of the receiver. Et will be noticed, however, that l2 is positive when mu is low, and negative when mu is high. Consequently, if l2 is considered to he the current produced by the receiver, and mu the permeability of the transmitter, the current la produces more field in the transmitter when le is negative than when Yit is positive. Hence, ii the receiver magnets producing l2 are displaced angularly from the transmitter magnets, the net fieid produced by I2 in the transmitter causes a torque to be developed in the transmitter. Similarly, a torque is produced in the receiver at the same time. Moreover, any attempt to change the value of the 'torque in the transmitter by simply changing the magnitude or the magnets or second harmonic impedance results in substantially the same percentage change in the torque of the receiver.

However, if phase sluiting impedances Z1 and Z2, such as resistances, are added to the second harmonic circuits, either in the windings or as special circuit elements as indicated in Fig. 2, so as to make the impedance power factor angie, for example, 45 degrees, and an impedance ZE, such as a resistance, is also added to theexcitation circuit .of the receiver so as to make the phase angle of the receiver excitation current 221/2 degrees on the fundamental base, or 45 degrees on the second harmonic base, ahead of the transmitter exciting current, then the current relations shown in Figs. 10 and ll resuit.

In Fig. 10 the permeability mu of the receiver has been advanced in phase due to the increase in power factor of the exciting winding circuit of the receiver and no longer coincides with the permeability mu of the transmitter. Also, due to the improvement in power factor 0f the coupling winding circuits, It of the transmitter now lags the voltage E2 of the transmitter by only 45 degrees. The advance in phase of mu of the receiver causes the coupling winding voltage thereofA to be advanced in phase and is represented by E'n, Fig. 11, and hence, is advanced 45 degrees ahead of the voltage E2 of the transmitter. 'I'he combined coupling Winding current of the receiver may now be represented as I'n, Fig. 11; only 45 degrees behind the voltage, again due to the improvement; in power factor oi the coupling winding circuits.

It will now be noted from Fig. 10 that the current I: of the transmitter is 180 degrees out of phase with the permeability mu' of the receiver. Hence, such current produces torque in the receiver. However, the current I of the receiver is 90 degrees out oi' phase with the permeability mu of the transmitter, and the average permeability mu is the same during the positive halfcycle of I's asv during the negative half-cycle. Hence, the receiver current Ig again produces no torque in the transmitter regardless of the rotor magnet positions. This explanation assumes that Ia is the resultant current pattern produced in all oi the coupling windings of the transmitter and that it flows in the receiver, and that I'z is the resultant current pattern produced in all of the couping windings of the receiver and that it flows in the transmitter. These currents are of course imaginary and do not actually exist, but the flux and torque results are the same as if such currents existed.

If @zz represents the power factor angle of the impedance of the second harmonic, circuit, considered positive when leading, and c represents the phase angle between the exciting current of the transmitter core and that of the indicator core, considered positive when the transmitter exciting current ieads the indicator exciting current, then the condition for zero torque on the transmitter is Where n is any integer including zero; and the condition for maximum torque on the indicator is The relation satisfying both of these two equations exactly gives 0zz=i45. However, the Equation I is the only one which is very critical because the second being the condition for a maximum, the rate of change of torque with respect to angle at this point is zero. Hence, it can be said that any relation of om and 20e which very nearly satisfies Equation I and approximately satisfies Equation II will produce the desired results. Consequently, a system is thus provided consisting of a transmitter and a receiver in which the receiver is urged to follow the transmitter in angular relation by an electro-magnetic coupling torque which is a function of the displacement angle of the receiver relative to the transmitter, said system, however, producing substantially no tendency for the transmitter to be affected by the angular position of the receiver.

Obviously, other relations between exciting cur` rent, phase angles, and power factor angle of the second harmonic circuit will produce zero "feedback torque, but the 45-degree relation explained is very satisfactory, and the reduction in receiver torque caused by increasing the impedance of the second harmonic circuits is not great. In fact, series capacitors in the second harmonic circuits would accomplish the same phase angle result and at the same time, increase the torque on the receiver. Also, obviously, capacitors could be put in the indicator excitation circuit to make much stronger than usual and serve for the additional purpose of a transmitter rotor, the stator element of which may tip relative to the compass magnets in order that the apparatus may be used on an airplane and the like and still transmit a true compass indication, even when the stator is tipped from a level position. To obtain this result and have the compass magnets iree at all times to respond and line up with the horizontal component of the earths magnetic field, it is necessary that the magnetic iniiuence on the compass magnets tending to produce rotation of both the stator core and windings of the transmitter be made symmetrical under the various conditions and relative positions encountered. This is accomplished by using the adjust-able magnetic strip ilii, if necessary; by a proper positioning of the compa-ss magnets, the pivot, and the ring core of the transmitter, as explained previously, to avoid certainrotary positions of different reluctance between magnets and core when the latter is tipped, and to avoid errors associated with the vertical component of the earths magnetic neld, to overlap the coupling windings of the transmitter so that current iicw therein will not produce rotary torque or locking tendency on the compass magnets, and iinally to adjust the power factor relations of the electrical system so that torque is transmitted only from transmitter to receiver and not from receiver to transmitter.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative and that the invention maybe carried out by other means.

What I claim -as new and desire to secure by Letters Patent of the United States, is:

l. In a remote indicating system, an electrical transmitter having a ring-shaped magnetic core, a distributed exciting winding thereon and separate coupling coils at dierent positions on said core for transmitting position signaling currents, a rotor for said transmitter which is positioned in accordance with the indication to be transmitted and having permanent magnet means for producing a unidirectional field in the stator to vary the currents in the coupling coils thereof in accordance with rotary movements of the rotor, said coupling coils being overlapped for the purpose and to the extent necessary to prevent locking torques on the rotor.

2. In a remote indicating system, a signaling device of the saturable core second harmonic type comprising a stator having a ring-shaped magnetic core. an alternating current exciting winding uniformly distributed thereon and separate quadrature spaced coupling coils on said core, a rotor cooperating with said stator having permanent magnet means for producing a unidirectional field diametrically oi said stator, said quadrature spaced coupling coils each extending over an arc ci degrees of said stator core energies so as to overlap for the purpose of preventing' locking torque on the rotor.

3. A telemetering system comprising a transmitter unit and a receiver unit, each unit comprising a. stator member and a rotor member, each stator member comprising a ring core of saturable magnetic material carrying winding means thereon provided with poiyphase secondharmonic connections and single phase excitation connections and each rotor member comprising a rotatable, poled permanent magnet, circuit means for connecting the single phase connections of the transmitter and receiver stator winding means to a common source of periodically varying current, circuit means interconnecting the polyphase connections of the transmitter and receiver stator winding means to permit iiow of induced, second-harmonic currents between the transmitter and receiver units. a

'irst phase-shifting impedance means electricaliy connected in the excitation circuit of one ci. said units to shift the relative phase angle of the transmitter and receiver excitation currents, and a second impedance means electrically connected in the circuit including said polyphase connections to shift phase angle of second harmonic currents flowing between transmitter and receiver units, said. irst and second impedance means being dimensiciied so that the resulting phase shift causes the values ci the transmitter generated second-liars monic currents iowing in the receiver approri met-ely tc correspond in time phase with thed minmum anni maximum values of permeability in receiver core permeability variation cycle and the maximum and minimum values oi receiver generated second-harmonic currents owing the transmitter to be displaced approximately 90 electrical degrees in time phase from the marde rnum and minimum values of permeability in the transmitter core permeability variation cycle whereby rotation oi the transmitter magnet is effective in producing in the receiver a torque tending to rotate the receiver magnet in angular correspondence with the transmitter magnet but rotation ci the receiver magnet produces no feedback torque in the transmitter tending to rotate the transmittermagnet.

s. In a self synchronous tron system, transmitter and receiver umts each having simile phase excitation circuit connected to a common runnin and minimum source of periodically varying current and polyphase circuit connections, circuit means for interconnecting the polyphase circuit connections of the transmitter and receiver units for transmission oi signal currents therebetween, rst phase-shifting impedance means electrically connected in the excitation circuit of one of said units to shift the .relative phase of the excitation currents in the transmitter and receiver units and second phase-shifting impedance means connected in the circuit of said polvphase connections to shift the phase of the signal currents flowing between the transmitter and receiver units, said impedance means being dimensione'd to cause approximately a 90 degree phase shift in the signal currents iiowing from the receiver unit to the .transmitter unit and approximately a li5 degree phase shift in the same direction of both the ,signal currents flowing from the transmitter unit to the receiver unit and the receiver unit excitation current relative to the transmitter unit excitation current whereby 'the transmitter is eective to control the receiver but the receiver exerts no inuence on the transmitter.

.iLLEN T. SXNKS.

.REBEBEENGES MUSEE) The following references are of record in the le oi this patent:

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Number riante Date 1,3.91553 Hall Mar. 3l, i914 ifiZi' Bristol Get. lo, 1922 ifii Blanchard May i0, 192'? 2,053fif Uriel' Sept. 3, 1936 2,196,809 Headley Apr. 9, 19e@ 2248,?@8 Draper et al. July 8, 194i 2,25 Stuart Nov. fr, 194i 2,308,566 iiloxon Jan. 19, 1%3 2,313,582 Stuart Mar. 9, i943 2,323,566 Peterson July 6, 1943 2,342,637 Bechhera'er Feb. 29, 1944 2,356,186 Somers Aug., 222 194e 2,361,590 Nexen Oct. 3l, 194i FOREIGN PTENTS Number Country Date 98,414 Sweden Mar. 19, 1940 

